This invention was made by an employee of the United States Government and may be manufactured and used by or for the Government for government purposes without payment of any royalties thereon or therefore.
1. Field of the Invention
The present invention relates generally to aromatic polyimides. It relates in particular to a heat, moisture, and chemical resistant thermosetting polyimide, which is especially useful as an advanced coating, adhesive, film, composite matrix resin, and neat resin.
2. Description of the Prior Art
In-situ Polymerization of Monomer Reactants (PMR) polyimides has significantly advanced the field of polyimides as processable engineering materials in the past three decades. PMR polyimides are presently used as composite matrix resins in various military aircraft engine components. Of particular importance are commercially available PMR 15 (U.S. Pat. No. 3,745,149), AFR 700B (U.S. Pat. No. 5,091,505) and LaRC RP 46 (U.S. Pat. No. 5,171,822). These PMR polyimides are prepared by dissolving a monoalkyl ester of 5-norbornene-2,3-dicarboxylic acid (nadic ester, NE), an aromatic diamine, and a dialky ester of an aromatic tetracarboxylic acid in a low-boiling alcohol (e.g., methanol or ethanol). The monomeric solution is then used to impregnate the reinforcing fibers. In-situ polymerization through the nadic end group occurs directly on the fiber surfaces, producing a composite material with excellent thermal and mechanical properties. LaRC RP 46 provides an example of how PMR polyimides are typically prepared (FIG. 1). Attractive features of the PMR process include the use of a low-boiling solvent and low molecular weight, low viscosity monomers. While these features are beneficial for composite processing, they create serious drawbacks for coating, adhesive and thin film applications.
One problem encountered is that the alcohol (methanol) solvent readily evaporates to produce a loose, dry powder. The dry powder can easily fall off from the surface of a substrate. More importantly, the polyimide prepared by the PMR process is very brittle and easily fractured. As FIG. 1 demonstrates, the brittleness is caused by the chemistry: NE and dimethyl ester of 3,3xe2x80x24,4xe2x80x2-benzophenonetetracarboxylic acid (BTDE) compete for 3,4xe2x80x2-oxydianiline (3,4xe2x80x2-ODA), and they react to form a mixture of salt complexes in an alcohol solution. NE reacts with the diamine, resulting in the formation of mononadimide and bisnadimide. In contrast, BTDE does not react with 3,4xe2x80x2-ODA to form a stable product in an alcohol solution. The formation of low molecular weight mononadimide and bisnadimide causes the polyimide resin to become brittle and reduces the average molecular weight between the crosslinking sites of the polyimide, producing a highly crosslinked and brittle network structure.
The brittleness and solvent evaporation problems can be solved by preparing a polyimide using a two-step, condensation methodology. When such a method is used, an anhydride and dianhydride react with a diamine in a high-boiling aprotic solvent to give a long molecular weight polyamic acid precursor through a step grow polymerization. This synthesis has multiple advantages over the PMR process for coating, adhesive and film applications. To begin with, the use of a high-boiling aprotic solvent will prevent evaporation of the solvent. Moreover, the polyamic acid solution is very tacky, which makes it easy to apply a coating, adhesive, and thin film to a substrate, and the chain extension in the solution is significantly easier than that of PMR polyimides in a molten state. In addition, because an anhydride and a dianhydride are significantly more reactive toward an aromatic diamine than their monoalkyl and dialkyl ester derivatives used in the PMR process, the reactivity difference between the anhydride and dianhydride is much smaller, compared to the reactivity disparity between the monoalkyl and dialkyl esters discussed previously. Finally, unlike the dialkyl ester, the dianhydride reacts rapidly with the aromatic diamine in an aprotic solvent, so that low molecular weight monoimide and bisimide do not form and cause the polyimide to become brittle. When the above referenced method is used, the resulting polyimide will have good flexibility and toughness, which are important properties for coating, adhesive and thin film applications.
The two-step condensation method is used almost exclusively for coating, adhesive and film applications. For example, U.S. Pat. Nos. 5,866,676, 5,147,966, and 5,741,883 disclose thermoplastic polyimides and U.S. Pat. No. 5,412,066 discloses a thermosetting polyimide, all of which were prepared by a condensation method and all of which exhibit excellent adhesive and film properties. However, they have low glass transition temperatures (Tg), which limit their use temperatures to 150xc2x0 C. to 200xc2x0 C. They also absorb a moderate amount of moisture (about 3%) and show poor resistance to hydraulic fluids, jet fuels, lubricating oils, common organic solvents, sea water and cleaning solutions.
Polyimides are prepared from mixtures of (a) 3,3xe2x80x2,4,4xe2x80x2-benzophenonetetracarboxylic dianhydride (BTDA), (b) 3,4xe2x80x2-oxydianiline (3,4xe2x80x2-ODA), and (c) 5-norbornene-2,3-dicarboxylic anhydride (NA) in a high boiling, aprotic solvent to give 5 to 35% by weight of the polyamic acid solution. The polyamic acid is converted into the polyimide by use of a heat treatment or by chemical cyclodehydration. The mole ratio of a:b:c is n:(n+1):2, wherein n has a value of 1.0000 to 50.4025, which corresponds to a formulated molecular weight of the uncrosslinked polyimide ranging from 978 to 25,000 gram per mole. These polyimides have excellent thermo-oxidative stability and resistance to moisture and chemical induced degradations. They also have outstanding mechanical properties.
The primary objective of the present invention is to provide what the prior art failed to produce, a family of polyimides having a desired combination of heat, moisture and chemical resistance that can be used as a specialty coating, adhesive, or thin film. These polyimides have the following properties:
(1) Use temperatures ranging from xe2x88x92100xc2x0 C. to 371xc2x0 C. that can withstand a sudden temperature surge of up to 600xc2x0 C.;
(2) Low moisture absorption (less than 1% after 24 hours in water at room temperature);
(3) Excellent chemical resistance to common organic solvents, hydraulic fluids, fuels, lubricating oils, cleaning solutions and sea water;
(4) Excellent adhesion to a wide variety of metallic and non-metallic substrates;
(5) They are relatively easy to process;
(6) Environmentally friendly (low toxicity); and
(7) Relatively low cost.
Another aim of this invention is to provide processes for producing the compositions and their useful end products. Yet another goal is to produce new products that are useful as coatings, adhesives, thin films, molded articles, prepreg, fiber reinforced composites, ceramic powder reinforced microcomposites, flexible printed circuitry, multifunctional smart materials such as sensors and actuators, and light weight foam materials.